1. Field of the Invention
The present invention relates to a method of operating a semiconductor device. More particularly, the present invention relates to a method of operating a multi-level cell (MLC) of a non-volatile memory.
2. Description of Related Art
Among various types of memory products, one non-volatile memory is a memory has widely used in personal computers and electronic products. In one non-volatile memory, data can be stored, read out or erased numerous times, and any stored data can be retained even after power is cut off.
A typical non-volatile memory is capable of storing two data states of “0” and “1”, and is a memory that storages one bit per cell (1 bit/cell). When reading data in the memory, a gate voltage is set to Vread. As Vread is larger than a threshold voltage (Vt) of the memory, a current flows through a source and a drain of the memory, and the data state is determined to be “1”. As Vread is smaller than a threshold voltage of the memory, don't have any current flows through the source and the drain of the memory, and the data state is determined to be “0”.
In recent years, with the development of high-density memory devices, each cell of the non-volatile memories can store more than one bit, i.e., the non-volatile memory is a so-called multi-level memory device. Each cell of this kind of memories can store multi-bit (two or more bits) data. Thus, the density of the stored data on the same chip area is increased. In order to store two or more bits of data in each cell, each cell is programmed to 22 levels, i.e., 4 levels. Here, the threshold voltages of the 4 levels are corresponding to 4 storage states of “00”, “00”, “10”, and “11” respectively.
However, as each cell in the multi-level memory device is programmed, the amount of injected electrons cannot be controlled precisely. Thus, the Vt-distribution curve of cells in various storage states is broad, which easily causing error in the reading. Moreover, the programming step that controls the threshold voltages in time mode, so that it is difficult to precisely obtain the target programming Vt.
The problem of the cells described above may influence the device performance, and reduce the reliability of the device. Therefore, it has become an important subject of the industry to solve this problem.